Through this proposal the PI seeks to develop an advanced MRI technique to quantify regional lung perfusion-ventilation ratios, and to use this technique to further understanding of ventilationperfusion mismatches. This proposal will test the Hypothesis that the extreme sensitivity of hyperpolarized (HP) MRI can be used as a high-resolution, indirect detector for lung ventilation-perfusion (V/Q) mismatches. A secondary hypothesis is that HP-gas MRI will allow for quantification of V/Q mismatches and that the new method can be used to establish the physiological significance of sub-segmental emboli. Specific Aims: 1). Quantify the sensitivity of HP-gas MRI to indirectly detect changes in susceptibility and derive relative flow using established tracer kinetics models. 2). Measure changes in alveolar HP 3He phase and T2* maps of the lungs of healthy pigs before and after intravenous injection of gadolinium. 3). Evaluate the capabilities of HP-gas MRI for detecting segmental and sub-segmental ventilation-perfusion mismatches in the case of pulmonary emboli. Significance: Quantification of regional ventilation and perfusion is of paramount importance for the diagnosis and treatment of a variety of pulmonary disorders, including pulmonary embolus, asthma, emphysema, and interstitial lung diseases. Accurate diagnosis of V/Q mismatches requires high-resolution visualization of regional lung perfusion and ventilation. The proposed method for quantifiable V/Q MRI will be a powerful tool in understanding the complex mechanisms acting during various acute or chronic lung diseases. The results will extend our abilities to detect and diagnose lung diseases, which are the 4th leading cause of death in the US.